var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];

function setArray(arr1, arr2) {
  let array = new Array();
  for (var index in arr1) {
      array.splice(index * 2, 0, arr1[index]);
      array.splice(index * 2 + 1, 0, arr2[index]);
  }
  return array;
}

let array = setArray(arr1, arr2);
console.log(array);
